1. Field of the Invention
This invention relates to solutions for forming phosphate conversion coatings on metal substrates, and particularly to such solutions containing an accelerator composition, as well as a method for the use of such solutions.
2. Statement of the Related Art
It is known in the art to form phosphate conversion coatings on metal surfaces, including iron, zinc, aluminum, and their various alloys such as steel, for the purposes of improving corrosion resistance and providing an adhesion base for subsequent coatings.
British Pat. No. 983,924 describes phosphating solutions containing zinc phosphate and manganese phosphate with an oxidizing accelerator composition comprising nitrate ions, and optionally nitrite ions. Other, optional, ingredients include tartaric acid and bromide ions. The oxidizing accelerator is particularly useful in the coating of ferrous surfaces, in which instance ferrous ions enter the phosphating solution and are oxidized to ferric ions, which results in a more consistent coating. A total acidity of greater than 20 points is disclosed, although the only working example has a total acidity of 30 points. The temperature of the phosphating solution was ambient (22.degree. C.), and the metal surfaces were sprayed with the solution for 1 minute.
German published patent application No. 28 18 426 describes a phosphating coating solution containing boron fluoride and using nitrate ions as the oxidation accelerator, optionally combined with chlorate and/or nitrite ions. The solution is particularly useful for coating aluminum surfaces. The disclosed compositions appear to have poor storage stability, and it is believed that this would require replenishment or adjustment of the phosphating solutions before each use.
British Pat. No. 1,542,222 describes phosphate conversion coating solutions containing zinc and phosphate and oxidizing acceleration compositions containing as essential ingredients, chlorates and sodium nitrobenzene sulfonate (SNBS). Nitrate ions are preferably also present. In the disucssion of prior art, mention is made of various proposed accelerators for zinc phosphate coating solutions, including organic nitro compounds (such as SNBS), nitrates, nitrites, hydrogen peroxide, and chlorates, bromates, and iodates. There is no further disclosure of bromates, although reference is made to British Pat. No. 828,916 for a zinc phosphate coating solution containing calcium.
Published German patent application No. 24 18 118 describes a conversion coating solution containing phosphate ions, and a wetting agent/possibly accelerator which can be a bromate or nitroorganic. Phosphating is carried out in two stages, in which the phosphate ions are increased and the wetting agent is decreased in the second stage.
Published German patent application No. 22 32 067 and corresponding Belgian Pat. No. 785,828 discloses a conversion coating solution containing zinc ions and phosphate ions in a 1:12-100 ratio, which ratio is used to regulate the coating quality. Nickel ions, manganese ions, or calcium ions may be added to provide further acid resistance.
Other prior art describes attempts to improve phosphating processes with respect to their efficiency, controllability, storage stability, and galvanoplastic features. For example, a rapidly acting secondary oxidant may be employed in an amount sufficient to oxidize ferrous ions to ferric ions, but not in an amount sufficient to have a dipolarizing effect and act as the primary oxidizing accelerator. Suitable rapidly acting secondary oxidants include alkaline metal or ammonium nitrate, hydrogen peroxide, compounds which release hydrogen peroxide under acid conditions, sodium hypochlorite, perphospates and peborates.
All of the prior art phosphating solutions and processes have certain serious problems. The use of nitrites, which evolves noxious fumes, poses obvious environmental dangers. Moreover, effluent containing nitrites are particularly dangerous and require treatment before disposal.